MILESTONES OF PHYSICS LEADING  TO NUCLEAR POWER

1895-1954

Draft of the essay for the project "Matter, Energy and People", sub-project "Energy from Fission"

Part I : A short history of fission and nuclear power development (a perspective of Nobel prize)

Part II : How a modern nuclear reactor works (coupling to visual presentation done by Bartek Gudowski & Andreas Harju)

Part III : How can future nuclear power look like: a transmutation reactor (to be developed based on existing visual presentation and its further development)

 

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SITUATION BEFORE 1895:

The turn of the century marked a profound revolution in the development of science and our understanding of the fundamental principles of the natural world. During the nineteenth century classical physics - the laws of motion, electromagnetic fields, and thermodynamics - had reached an advanced state of development.  The picture of a physical world seemed to be well understood based on these laws.  To some it seemed that  physics was reaching such a state of maturity that few fundamental principles remained to be discovered. Also chemistry had reached a considerable degree of sophistication but on a largely empirical basis, the fundamental basis of chemistry remained mysterious. Much had been learned about the Earth and solar system as well. Estimates of the age of the Earth had risen from about 6000 years in the late eighteenth century to tens or hundreds of millions of years; and the view that life, the Earth, and the rest of the solar system had arisen in a single great upheaval in recent times had been replaced by the idea of gradual change over years.

Physicists at the end of the nineteenth century believed that most of the fundamental physical laws had been worked out. They believed that atoms (if they exist!!) consist of hydrogen atoms (approximately) and they expected only minor refinements to get ``an extra decimal place'' of accuracy.

But there were problems. Essentially nothing was known about the fundamental structure of matter that gave rise to the Periodic Law and other chemical behaviors - the very existence of atoms was largely conjectural. Geology and astronomy seemed in serious conflict since the apparent age of the geologic record could not be reconciled with the only power source for the Sun then conceivable, gravitational contraction, which would exhaust itself in mere millions of years. An important part of classical thermodynamics was stubbornly resisting resolution - the properties of blackbody radiation. In fact by the end of 1900s it had become clear that within the existing framework of physics no solution of the blackbody problem was possible (the untenable prediction made by existing physics was termed the "ultraviolet catastrophe"). Something important was missing. And soon the inadequacy of classical physics was proofed.

In 1895 new experimental results in physics have profoundly shaken the very foundation of the XIX-century science. Henri Becquerel, professor at the National Museum of Natural Sciences in Paris, inspired by an earlier discovery of so called X-rays done by Wilhelm Conrad Roentgen, discovered that an unknown penetrating radiation was emerging from  uranium salt (the potassium uranium sulfate). Uranium made its remarkable "entré" as an extremely interesting research material.

 

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The discovery of  Becquerel had inspired a young Polish scientist, Maria Sklodowska, who just moved from Warszawa to Paris and married an oustanding French physicist and a great discoverer - Pierre Curie, to study carefully these new rays emitted from uranium. It was Maria who  coined term radioactivity. After chemical extraction of uranium from the ore (pitchblende), Maria noted the residual material to be more “active” than the pure uranium. She concluded that the ore contained, in addition to uranium, new elements that were also radioactive. This led to their discoveries of the elements of polonium and radium, but it took four more years of processing tons of ore under oppressive conditions to isolate enough of each element (about 1 gram) to determine its chemical properties.

For their work on radioactivity, the Curies were awarded the 1903 Nobel Prize in physics.  To honor Curies, the 1910 Radiology Congress chose the curie as the basic unit of radioactivity: the quantity of radon in equilibrium with one gram of radium (current definition: 1 Ci = 3.7x10¹⁰ dps).  A year later, 1911, Maria was awarded the Nobel Prize in chemistry for her discoveries of  radium and polonium, thus becoming the first person to receive two Nobel Prizes. For the remainder of her life she tirelessly investigated and promoted the use if radium as a treatment for cancer.

Discovery of radioactivity began the transition from XIX-century science” to a revolutionary new way of looking at nature, a paradigm change, which was completed in 1903, when Ernest Rutherford and Frederick Soddy explained radioactivity as the spontaneous transmutation of elements. Ernest Rutherford, who came from New Zeeland to Cavendish Laboratory i Cambridge, England, and then moved to the McGill University in Montreal became a father of nuclear physics. Indeed, it could be said that Rutherford invented the very language to describe the theoretical concepts of the atom and the phenomenon of radioactivity. Particles named and characterized by him include the alpha particle, beta particle and proton. Even the neutron, discovered by James Chadwick almost 30 years later, owes its name to Rutherford. The exponential equation used to calculate the decay of radioactive substances was first employed for that purpose by Rutherford and he was the first to elucidate the related concepts of the half--life and decay constant.

 For this work, Rutherford won the 1908 Nobel Prize in chemistry.

In 1909, at the University of Manchester, Rutherford was bombarding a thin gold foil with alpha  particles when he noticed that although almost all of them went through the gold, one in  eight thousand would “bounce” (i.e., scatter) back. The amazed Rutherford commented that it was “as if you fired a 15--inch naval shell at a piece of tissue paper and the shell came right back and hit you."

From this simple observation, Rutherford concluded that the atom's mass must be concentrated in a small positively--charged nucleus while the electrons inhabit the farthest reaches of the atom. Although this planetary model of the atom has been greatly refined over the years, it remains as valid today as when it was originally formulated by Rutherford.

In 1919 Rutherford showed that high-energy alpha particles could cause an alteration in the nucleus of an ordinary element. Specifically he succeeded in changing a few atoms of nitrogen into atoms of oxygen by bombarding them with alpha particles. Nitrogen + helium became hydrogen + oxygen. A nuclear reaction was discovered!!

About 14 years before the discovery  of nuclear reactions, in 1905 Albert Einstein published 3 papers of great importance for modern physics. In the first of those three papers Einstein examined the phenomenon discovered by Max Planck, according to which electromagnetic energy seemed to be emitted from radiating objects in discrete quantities. Einstein used Planck's quantum hypothesis to describe the electromagnetic radiation of light.

Einstein's second 1905 paper proposed what is today called the special theory of relativity. He based his new theory on a reinterpretation of the classical principle of relativity, namely that the laws of physics had to have the same form in any frame of reference. As a second fundamental hypothesis, Einstein assumed that the speed of light remained constant in all frames of reference, as required by Maxwell's theory.

Later in 1905 Einstein showed how mass and energy were equivalent. The famous equation E=mc² caught the imagination of physicists and even journalists all over the world. The foundation of the nuclear energy, equivalence of mass and energy, was formulated in one simple equation, which explained how in one single nuclear reaction energy can be realesed in quantities exceeding million times more than in a single chemical reaction.

Einstein received the Nobel Prize in 1921 but not for relativity or mass-energy equivalence but for his 1905 work on the photoelectric effect. In fact he was not present in December 1922 to receive the prize being on a voyage to Japan.

Thanks to Rutherford experiments and remarkable contribution of Niels Bohr the atomic structure appeared to be understood in 1920's, however, the basic structure of an tomic nuclei was still unknown. It was rather clear that a nuclei had to contain a number of protons, but to get a right charge it had to be assumed that electrons were also a part of nuclei or an unknown, neutral particle was contained in an atomic nuclei. Rutherford had many times suggested existence of a neutral particle - a neutron - but one had to wait until 1932 to witness the discovery of this particle. In 1932 James Chadwick, Rutherford´s pupile, proved the existence of the neutron showing that berullium, when exposed to an alpha particles, gave off a particle which was not deflected by electric or magnetic forces. The particle was thus uncharged, yet it was massive enough to knock protons out of a sample of paraffin (which is rich in hydrogen). This discovery earned him the 1935 Nobel Prize in physics.

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In 1931 a daughter of Marie Sklodowska-Curie, Iréne married to Frédérick Joliot was very close to discovery of a neutron, but she and her husband misinterepretd the same experimental results which insired James Chadwick to prove the neutron existence. Fortunetely, a Joliot couple got their chance when they few years later, discovered an artificial radioactivity winning a 1935 Nobel prize in chemistry. They came to Stockholm together with James Chadwick,  to get their Nobel prizes, Chadwick in physics, couple Joliot - in chemistry.

In Italy, since 1920 a young genius in physics, Enrico Fermi, from 1926 professor in physics at Rome univesrsity, was conducting his research in just newly emerged radiation and nuclear physics. He immediately realized looking at Chadwick´s and Joliot-Curie's discovaries that the "neutral" neutrons would be ideal particles for interaction with nuclei and for creation new radioactive elements. He and his research team produced by neutron irradiation already in 1934 large quantities of radioactive elements. Their source of neutrons was beryllium powder enclosed with radioactive radon gas in a glass tube.

Two of Fermi´s discoveries have had specifically big impact on development of nuclear and neutron physics:

Discovery that fast neutron emerging from a neutron source are slowed down while passing throug hydrogen rich materials (like water or paraffin) until they reach thermal equilibrium with surrounding matter, neutrons are becoming "thermalized".  Those thermal neutron have much higher probability of interaction with nuclei - up to 100 times - than fast neutrons (expressing this in more scientific terms one can say that reaction cross-sections for thermal neutrons are much bigger than those for fast neutrons)

 

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Discovery that neutron irradiation of uranium lead to absorption of the neutron and creation of a new element heavier than uranium itself . However, Fermi´s conclusions were still uncertain.

 For these discoveries Fermi was honored with Nobel Prize in 1938.

Many laboratories in Europe followed intently new discoveries in neutron physics. Particularly a research pair Otto Hahn and Lise Meitner in Kaiser Wilhelm Institute in Berlin and a couple Joliot-Curie in Paris seemed to be most competent and suited to follow the secrets of neutron irrdaiation of uranium . Both groups had the highest competence in physics and chemistry.

During years 1935 - 38 both groups did a lot of experiments with irradaition of uranium. In 1938 couple Joliot-Curie was very close to realize what was happening during these irradiation. They reported that they could find traces of radioactive lanthanum-like elements in the neutron irradaited samples of uranium. But they never concluded decidedly that it was LANTHANUM - it seemed to be very improbable for them to find suddenly in uranium sample an element from the middle of the periodic system.

Lise Meitner, who did similar experiments with Otto Hahn in Berlin was forced to leave Germany in 1938 becasue of her a jewish origin. Consequently, Otto Hahn had to continue neutron irradiated uranium sample analysys with another collaborator, a young researcher Fritz Strassman. And it was just a team of highly qualified chemists : Hahn and Strassman, who found undoubtly barium in uranium sample. Otto Hahn informed immediately about his discovery his former collaborator, Lise Meitner, who was then living as a refugee in Kungälv in Sweden. Lise Meitner after some discussions about this sensational discovery with her nephew, physicist Otto R. Frisch, came to conclusion that uranium nucleus must have been splitted into two parts with an energy release exceeding 100 million times more than a usual energy release for chemical reactions.  A term FISSION was born. Uranium nuclei had been fissioned during neutron irradiation. Theoretical work done by N. Bohr and A. Wheeler suggested that it was only one uranium isotope, ²³⁵U, which can be fissioned by thermal (slow) neutrons.

Only Otto Hahn alone was awarded in 1944 a Nobel Prize for a fission discovery.

E. Fermi had left Italy and Europe in 1938, travelling directly from Nobel-prize ceremony in Stockholm to United States. He followed the path of thousands of European intelectuals (most of them of Jewish origin) who left Europe on the edge of the Second World War. Fermi was very warmly wecomed at Colombia University in New York and continued his extremely interesting and important research on uranium.

 

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The sensational discovery of fission in a uranium isotope triggered a scientific revolution. However, to convert this scientific revolution into a technical revolution, it should be possible o release fission energy in macroscopic quantities, not only in micrscopic ones, as in a single fission process. The fissian - a "nuclear flame" - should be possible to set on and sustain as easy as a match can light the fire . A condition for this self sustaining energy generation from fission was clear from the very beginning.  Only one neutron was needed to trigger a single fission of ²³⁵U, so if more than one neutron was being released in the fission process - a chain fission reaction could be possible. "Nuclear flame" could be set on in a macroscopic scale in enormous quantities.  A long time dream of generation of physicist was coming to happen: inner energy of atomic nuclei could be release for good of people. For good, and for bad - as it turned out.

In March 1939 two reserach groups showed indepedently that few neutrons were liberated during fission process. First was F. Joliot with two collaborators: Hans von Halban and Lev Kowarski, who estimated  that about 3 - 3.5 neutrons are liberated in a single fission process (a corrcet number of neutrons liberated in a slow neutron induced fission of ²³⁵U is 2.43) . Shortly after E. Fermi at Colombia University confirmed this discovery. A hunt for nuclear energy has began, just at the time when the Second Worl War was to explode in Europe. Very soon uranium research became a centre of interest not only for devoted scientists but also a topic of world politics: a brilliant scientist (not only physicist but also biologist and nuclear engineer), Leo Szilard  realised that fission process can be used for a construction a mass-destruction weapon. Together with his colleage E. Wigner they convinced A. Einstein to sign a letter they drafted to American president F.D. Roosevelt, informing him about a possibility of weapon construction based on fission chain reaction in uranium or other heavy isotopes. A danger was that nazi-Germany was already trying to develop such a bomb. To avert this danger USA decided to join the race for nuclear weapon.

Parallely to discoveries in neutron induced fission in Berkeley, California E. Lawrence has constructed an "atom smashing" machine - a cyclotron. On this machine have new elements and isotopes havier than uranium, been discovered: E. McMillan discovered neptunium, and G. Seaborg - plutonium. All three men were honored by Nobel committee.

 

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Investigations have shown that plutonium is even better fissile material than ²³⁵U.

To be continued in the next version...

Essay by:

Waclaw Gudowski, PhD

Department of neutron and reactor physics

Royal Institute of Technology

Sweden

é-mail: wacek@neutron.kth.se

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